"Disc rot" isn't really rot, but it is a real thing. If the aluminum layer in the center of the disc is exposed to air, water, etc. it will corrode. This doesn't mean "Oh meh the gawdz!!!! My games r rottin!" It means that like everything else, it won't last forever.
Like I said, taking care of games is the best way to ensure they don't crap out. But you don't know how somebody else treated their games. You don't know what conditions they were stored in . Especially if you are buying used. Even more so if you are buying ratty-ass disc-only games that have been tossed around, slapped in sleeves, stepped on and left in storage sheds and yard sale tables.
I haven't personally had an issue with disc rot, but I have seen discs it has happened to. If that center aluminum layer is exposed and begins to corrode, you will see the tell-tale "burn" or pinholes in it.